The present invention relates to a method for controlling the air-fuel ratio of an internal combustion engine, and more particularly to a method for controlling the air-fuel ratio of an engine capable of optimum control without the waste of time heretofore inherent with the use of an oxygen sensor.
In controlling the air-fuel ratio of the engine, generally, an oxygen sensor (hereinafter referred to as an O.sub.2 sensor) is used as a gas sensor. This O.sub.2 sensor is attached to an internal combustion engine so as to detect the oxygen concentration in the exhaust gas from the engine and thereby control the air-fuel ratio. The output of the O.sub.2 sensor is detected by the oxygen concentration in the exhaust gas. It is well known that there is a time deviation or delay between the air-fuel ratio or mixture at an instant of taking the mixture into the engine and the air-fuel ratio at an instant detected by the O.sub.2 sensor after burning the mixture in the engine cylinder. FIG. 1 shows an excess air ratio .lambda. and a transient response of the O.sub.2 sensor. FIG. 1 shows a condition wherein the excess air ratio .lambda. is changed in step fashion between 0.9 and 1.1. T.sub.S is a delay time of the system previously determined by the engine, that is, a time required for the fuel to reach the O.sub.2 sensor after being burned in the engine cylinder when the excess air ratio .lambda. is changed between .lambda.=1.1 (lean air-fuel ratio) and .lambda.=0.9 (rich air-fuel ratio). T.sub.R and T.sub.L are time delays of the O.sub.2 sensor itself when the air-fuel ratio is changed from the lean condition to the rich condition and from the rich condition to the lean condition, respectively. The conventional method of controlling the air-fuel ratio, taking these time delays into consideration, is illustrated in FIGS. 2a and 2b. That is, FIG. 2a shows a relation between the change relative to time of the air-fuel ratio and a comparison voltage of the O.sub.2 sensor. FIG. 2b shows a relation between the excess air ratio .lambda. and the controlled output of the device relative to time. It has been found that FIGS. 2a and 2b correspond as to time with each other. As seen from FIG. 2a, when the output of the O.sub.2 sensor is changed in turn from a lean to a rich condition (at X.sub.1) and a rich to a lean condition (at X.sub.2) the performance curve of the air-fuel ratio is intersected each time by the comparison reference voltage level (0.5 V) formed by the O.sub.2 sensor. As shown in FIG. 2b, when control of the sensor output, shown by a solid line, operates toward or to the lean in the case of rich condition of air-fuel ratio, and toward or to the rich condition in the case of lean condition of air-fuel ratio, a time delay or control deviation in time is generated. At the intersection of the sensor output with the comparison voltage 0.5 (at the inversion), a step gain shown by a dotted line is added to an integrating gain (solid line in FIG. 2b), thereby resulting in an adjustment of control amount with a time correction. That is, the amplitude of oscillatory control is controlled.
In the above conventional method, however, when output of the O.sub.2 sensor is frequently intersected with the comparison voltage of 0.5 V, i.e., when the air-fuel ratio is frequently changed, each time the step gain shown by the dotted line is added, stable control cannot be obtained, and the wasted time cannot be eliminated.